Disable most of OnPropertyChanged logic for non-shared models.
Disable most of OnPropertyChanged logic for non-shared models.

Automatic validation, dirty marking and MessageBus broadcast on
OnPropertyChanged only happens if the model IsShared now.

This change shaved off around 50% of the time spent in the method in
overall.

peeedge/mobile,ZhangLeiCharles/mobile,eatskolnikov/mobile,masterrr/mobile,eatskolnikov/mobile,peeedge/mobile,ZhangLeiCharles/mobile,eatskolnikov/mobile,masterrr/mobile